Abstract: The results of elastic modulus measured by unilateral extensometer and bilateral extensometer were compared by using tensile test machines with different coaxiality. The results show that the coaxiality had a great influence on the measurement of elastic modulus of metal materials, especially on the measurement of elastic modulus with unilateral extensometer. It is suggested to use bilateral extensometer to measure elastic modulus. It is suggested that the coaxiality of tensile test machine should be fully considered when GB/T 228. 1-2010 is revised.